The purpose of this retrospective study is to determine if the patient characteristics of birth weight (BW) and gestational age (GA) or the physiological characteristics of heart rate (HR), long term variability (LT-HRV) or severity of illness scores independently predict mortality in premature neonates who underwent thoracotomy surgery while in the intensive care nursery (ICN). A secondary purpose of the study is to define the relative contribution of these factors to the explanation of mortality risk in premature neonates who underwent thoracotomy surgery. Status: Data collection finalized. Study completed.